This invention relates to metal oxide dielectric dense bodies, precursor powders and methods therefor, and multilayer semiconductor substrates made of the same.
Multilayer packaging in the electronics industry involves the use of ceramic powders which are sintered (or fired) to form a dense insulating (or dielectric) substrate for attaching semiconductor chips, connector leads, capacitors, resistors, and other electronic components. The substrate is first prepared as unsintered (or "green") sheets, onto which a desired pattern of electrical conductors (or precursor compositions therefor) is deposited by spraying, dipping, screening, etc. Interconnection between the various layers can be achieved by vias or feedthrough holes punched into them. The vias are metallized by filling them with a metal paste which, during the sintering process, is transformed into a sintered dense metal interconnection of a conductor such as copper. Thus, by superimposing a plurality of green sheets and interconnecting them with vias and sintering, a multilayer dielectric structure having a desired conductor pattern within it is prepared.
A common substrate is alumina (Al.sub.2 O.sub.3), which possesses a number of desirable characteristics, among which are high resistivity, high thermal conductivity, and good mechanical properties. However, alumina also possesses some limitations. Its dielectric constant is undesirably high, as is its thermal expansion coefficient. The high temperature required to sinter alumina powder into a dense body, approximately 1500.degree. C., is incompatible with the use of a highly conductive and inexpensive conductor such as copper (mp 1083.degree. C.). Instead, less conductive and/or more expensive conductors such as molybdenum or tungsten, having higher melting points, must be used.
An attractive alternative material to alumina is cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2), due in part to its low dielectric constant and thermal expansion coefficient. In a prior art preparation of dense cordierite, a mixture of the consitituents MgO, Al.sub.2 O.sub.3, and SiO.sub.2 is sintered at temperature in excess of 1300.degree. C. This method suffers from the same limitation regarding compatibility with copper conductors as the alumina method. Cordierite may be sintered to near-theoretical density by the glass-ceramic method at a lower temperature, about 1000.degree. C., which is compatible with the use of lower melting conductors such as copper, gold, or silver. In this method, a two-stage process is used. First, an appropriate composition (e.g., 13.78 wt. % MgO, 34.86 wt. % Al.sub.2 O.sub.3, and 51.36 wt. % SiO.sub.2) is prefired to a high temperature (about 1500.degree. C.) and then rapidly quenched to form a glass. The glass may include nucleating agents such as titanium oxide, zirconium oxide, phosphorus pentoxide, or stannic oxide. Second, the glass, after forming into a green body of appropriate shape, is heated to about 1000.degree. C. to form the dense body, with the nucleating agent helping to promote the crystallization of the glass. The ensuing dense body typically consists of fine grained crystals dispersed in a glassy matrix. Because copper conductor is applied after the first, higher temperature heating cycle this process is compatible with its use. Discussions of various aspects of glass-ceramic technology may be found in MacDowell, U.S. Pat. No. 3,275,493 (1966); Miller, U.S. Pat. No. 3,926,648 (1975); Kumar et al., U.S. Pat. No. 4,301,324 (1981); and Herron et al., U.S. Pat. No. 4,234,367 (1980). Prunier, Jr., in U.S. Pat. No. 4,745,092 (1988), discloses an alternative method of making cordierite having therein minor amounts of calcia from synthetic raw materials such as magnesium and aluminum salts and colloidal silica. However, his sintering temperatures are between 1380.degree. and 1440.degree. C., making his process incompatible with copper conductor.
Another alternative material to alumina is mullite (3Al.sub.2 O.sub.3.2SiO.sub.2), due in part to its low dielectric constant and thermal expansion coefficient. However, conventional preparation of mullite by sintering a mixture of the consitituents Al.sub.2 O.sub.3 and SiO.sub.2 requires a firing temperature in excess of 1350.degree. C. and therefore suffers from the same limitation regarding suitable conductors as alumina. Gardner, U.S. Pat. No. 3,826,813 (1974), discloses a process for making mullite for use as an integrated circuit substrate by a two-step process, in which the precursor materials are prereacted at 1300.degree.-1400.degree. C. and later sintered at 1500.degree.-1600.degree. C.
Yet another alternative material to alumina is magnesium oxide (MgO), due in part to its high thermal conductivity, which leads to efficient heat dissipation. However, the conventional preparation of a magnesium oxide dense body by sintering magnesium oxide powder requires a firing temperature in excess of 1350.degree. C. and therefore suffers from the same limitation regarding suitable conductors as alumina. De Jonghe et al., in J. Am. Ceram. Soc. 71, C-356 (1988), disclose another method of making magnesium oxide, by the liquid phase sintering of a magnesium oxide-bismuth oxide system. Bismuth oxide is added to an alcoholic suspension of magnesium oxide, and, after stir drying, the powder mixture is ground up with a mortar and pestle and sintered at about 1000.degree. C. to produce the dense body. However, the densification obtained is relatively low--only about 70-80% of theoretical.
The present invention provides novel methods of making dielectric dense bodies comprising cordierite, mullite, magnesium oxide, or other metal oxides, which are compatible with the use of copper or other low melting conductor because of their lower preparation temperature. There are also provided novel metal oxide dielectric compositions made by the process of this invention.